A characteristic neuropathological hallmark of Alzheimer's disease (AD) is the neuritic plaque, composed of an extracellular cluster of degenerating nerve terminals with a central core that is composed of deposits of a 4 kDa beta-amyloid peptide (beta-protein). beta-protein is derived from a much larger precursor protein (APP) with a size of between 95-135 kDa. Although the precise etiology and pathogenesis of AD are unknown, strong evidence for the involvement of APP in the cause of AD comes from recent findings in a small number of early onset AD families which carry a mutation on a locus on the APP gene which appears to segregate with the disease. The question of exactly how beta-protein is released from the precursor is of critical importance and remains unanswered. Based on evidence in our published work, we postulated that there were two separate and different processing pathways for APP: the secreted and intracellular pathways. Subsequent work in other laboratories has shown that the secreted pathway results in normal cleavage inside the beta-protein region, resulting in the release of the extracellular fragment of APP. This pathway is unlikely to be the source of intact beta-protein in the brains of Alzheimer's patients since roughly a third of the fragment remains buried in the nerve cell membrane. Our hypothesis, based on data accumulated in this last grant period, is that the source of intact beta-protein in the brains of patients with AD is an intracellular form of APP, since this processing pathway results in progressively smaller, amino terminally cleaved fragments which all span the entire beta-protein region. These may then be potential precursors for intact beta-protein. In this grant application we further extend our hypothesis by proposing possible mechanisms by which an intracellular pathway may arise for an integral membrane protein and propose experiments to test our hypothesis.